Deep remove specific empty json array in circe scala

Question

I want to remove deep empty json array from my json before/during processing by circe.

Incoming JSON

{
    "config": {
        "newFiles": [{
            "type": "audio",
            "value": "welcome1.mp3"
        }],
        "oldFiles": [],
        "channel": "BC"
    }
}

or 

{
    "config": {
        "newFiles": [],
        "oldFiles": [{
            "type": "audio",
            "value": "welcome1.mp3"
        }],
        "channel": "BC"
    }
}

Resulted Json should look like

{
    "config": {
        "newFiles": [{
            "type": "audio",
            "value": "welcome1.mp3"
        }],
        "channel": "BC"
    }
}

or 

{
    "config": {
        "oldFiles": [{
            "type": "audio",
            "value": "welcome1.mp3"
        }],
        "channel": "BC"
    }
}

What i understand that this can be done before decoding config as well as during decoding config. The idea here is i want to handle only one of files (either new or old) at my case class level.

Method 1: Tried at config decoding level which works well.

case class File(`type`: String, value: String)

case class Config(files: List[File],
                  channel: String = "BC")

object Config{
  implicit final val FileDecoder: Decoder[File] = deriveDecoder[File]
  implicit val ConfigDecoder: Decoder[Config] = (h:HCursor) =>
    for {
      oldFiles <- h.get[List[File]]("oldFiles")
      files <- if (oldFiles.isEmpty) h.get[List[File]]("newFiles") else h.get[List[File]]("oldFiles")
      channel <- h.downField("channel").as[String]
    }yield{
      Config(files, channel)
    }
}

case class Inventory(config: Config)

object Inventory {
  implicit val InventoryDecoder: Decoder[Inventory] = deriveDecoder[Inventory]
}

Method 2: Tried before feeding into decoding which didn't worked out

Let me know what could be the elegant approach to handle it. PS: I have multiple similar config decoders and if i handle this at config decoding level then there will be a lot of boiler plate code.

Luis Miguel Mejía Suárez · Accepted Answer · 2022-04-10 15:02:58Z

I have simplified the problem a little bit again, but combining this with the previous answer should be simple.

I also took advantage of cats.data.NonEmptyList

final case class Config(files: NonEmptyList[String], channel: String = "BC")
object Config {
  implicit final val ConfigDecoder: Decoder[Config] =
    (
      (Decoder[NonEmptyList[String]].at(field = "newFiles") or Decoder[NonEmptyList[String]].at(field = "oldFiles")),
      Decoder[String].at(field = "channel")
    ).mapN(Config.apply).at(field = "config")
}

This can be used like this:

val data =
"""[
{
"config": {
  "newFiles": ["Foo", "Bar", "Baz"],
  "oldFiles": [],
  "channel": "BC"
}
},
{
"config": {
  "newFiles": [],
  "oldFiles": ["Quax"],
  "channel": "BC"
}
}
]"""

parser.decode[List[Config]](data)
// res: Either[io.circe.Error, List[Config]] =
//   Right(List(
//     Config(NonEmptyList("Foo", "Bar", "Baz"), "BC"),
//     Config(NonEmptyList("Quax"), "BC")
//   ))

Note: I am assuming that at least one of the two lists must be non-empty and give priority to the new one.

You can see the code running here.

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